Default settings on the iPhone with iOS 5 or iOS 6 installed are set up so that when an incoming text or MMS message arrives, the device reminds you with two alerts. This repeated alert every 2 minutes can make sure you know a message came in, or it can be really annoying as you think the iPhone is receiving multiple SMS alerts in a row.
To switch off these reminder alerts, follow these steps:
1. Navigate to Settings -> Notifications -> Messages
2. Scroll to the bottom of the page and tap Repeat Alert
3. Select how many times you want to be reminded of each text message